Peak Flow - Modified Rational Method (LA County)

The Peak Flow - Modified Rational Method is an extension of the rational method used to create runoff hydrographs from a watershed of any size over a specific time period.The Rational method is limited to considering storms with a duration equal to the time of concentration and provides only a peak flow. The Modified Rational Method can consider single event storms with changing intensities and longer durations. It was developed as a means to produce hyrographs for storage design based on the rational method. Please refer to Chapter 12, Hydrology Manual of Los Angeles County Department of Public Works for hydrograph calculation.

Select Modified Rational Method (LA County) from Watershed > Peak Flow menu. In the dialog, specify the Drainage Area, Soil Type, Proportion Impervious Data, Time of Concentration, Storm duration and the Rational rainfall data. The drainage area can be obtained from picking a closed polyline and the program calculates the area automatically. Click on Calculate button to see the result.

The Soil Type library has all the tables from the Apprendix C in the Hydrology Manual. New Soil Type and Runoff Coefficient Data can be added to the library.

The Proportion Impervious Data library is from the Apprendix D in the Hydrology Manual. New data is allowed to add to the library.

Peak Flow Hydrograph:

Peak Flow Report:

Pulldown Menu Location: Watershed > Peak Flow
Keyboard Command: peakflw_modrat
Prerequisite: None